CODEWITHSUNDEEP
pythonarraysnumpy
user3228820
user3228820

Reputation: 21

strange behaviour of arange in python

I thought that in arange the second argument will never be comprised in the list, but see what happens in Python 2.7.3.

>>> import numpy
>>> numpy.arange(0.2,0.9,0.1)
array([ 0.2,  0.3,  0.4,  0.5,  0.6,  0.7,  0.8])
>>> numpy.arange(0.2,0.7,0.1)
array([ 0.2,  0.3,  0.4,  0.5,  0.6])
>>> numpy.arange(0.2,0.6,0.1)
array([ 0.2,  0.3,  0.4,  0.5])

but

>>> numpy.arange(0.2,0.8,0.1)
array([ 0.2,  0.3,  0.4,  0.5,  0.6,  0.7,  0.8])

Does anybody know the reason for this behaviour ?

Upvotes: 2

Views: 203

Answers (2)

jayelm
jayelm

Reputation: 7688

This is a problem with floating-point precision.

From the documentation:

stop : number

End of interval. The interval does not include this value, except in some cases where step is not an integer and floating point round-off affects the length of out.

To see more about the limitations of floats, look at Python's tutorial.

Upvotes: 4

M4rtini
M4rtini

Reputation: 13549

In [169]: val = 0.2

In [170]: for i in range(6): #simulating the loop in arange.
   .....:     val += 0.1
   .....:

In [171]: val
Out[171]: 0.7999999999999999

floating point rounding error. The result being that the last value is also printed.

Upvotes: 4

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